Water-regulated synthesis of Al2O3-SiO2 monolithic aerogel composites

Kurzfassung

Alumina-based mixed metal oxide aerogels are well known for their thermal stability. They are typically prepared by controlled hydrolysis of metal alkyl esters, subsequent condensation, drying and heat treatment. Recent progress in the synthesis of pure alumina aerogels involved the hydrolysis of aluminum tri-sec-butoxide (ASB) using acetone and aniline thus releasing water in a defined manner [1, 2]. In that context, we investigated into the ASB route for the synthesis of mixed alumina silica aerogels alternatively to existing routes which are based on aluminum chloride (AlCl3) and tetraethyl ortho silicate (TEOS) [3]. Within these studies it was revealed that water is still present after hydrolysis of TEOS and hampers homogeneous gel formation by a simple subsequently addition of ASB. To overcome this issue, we developed a new method featuring the addition of (non-toxic) trimethyl orthoformate (TMOF) to the silica sol for an irreversible removal of water before ASB was added. The homogeneous gel bodies were dried under supercritical CO2 conditions. One mayor advantage of the new developed method is the avoidance of toxic reagent. Within the presentation we will report on systematic studies of synthesis to achieve monolithic aerogel samples with varied molar ratios of Al:Si and Al:Si:solvent and the aerogel materials properties received after characterization with respect to shrinkage, density, thermal conductivity, mechanical properties, as well as microstructure by SEM and TEM. Furthermore, the effect of heat treatment (800 – 1200 °C) on the monoliths will be illustrated, too. All these properties are critical issues for the design of thermally resistant materials used for mobile and semi-stationary applications in aeronautics, space, and the transportation sector.